Wind Power Essay, Research Paper
The wind turbine, also called a windmill, is a means of harnessing the
kinetic energy of the wind and converting it into electrical energy. This
is accomplished by turning blades called aerofoils, which drive a shaft,
which drive a motor (turbine) and ar e connected to a generator. “It is
estimated that the total power capacity of winds surrounding the earth is
1 x 1011 Gigawatts” (Cheremisinoff 6). The total energy of the winds
fluctuates from year to year. Windmill expert Richard Hills said that the
wind really is a fickle source of power, with wind speeds to low or
inconsistent for the windmill to be of practical use. However, that
hasn’t stopped windmill engineers from trying. Today, there are many
kinds of windmills, some of which serve differen t functions. They are a
complex alternative energy source.
What to consider when building a windmill In choosing where to build a
windmill, there are many important factors to consider. First is the
location: 1) Available wind energy is usually higher near the seacoast or
coasts of very large lakes and offshore islands. 2) Available wind energy
is gene rally high in the central plains region of the U.S. because of the
wide expanses of level (low surface roughness) terrain. 3) Available wind
energy is generally low throughout the Southeastern U.S. except for
certain hills in the Appalachian and Blue Rid ge Mountains, the North
Carolina coast, and the Southern tip of Florida. This is because of the
influence of the “Bermuda high” pressure system, which is a factor
especially during the summer. Also important to consider is the wind
where you are going to build: 1) the mean wind speed (calculated my
cubing the averages and taking the mean of the cubes) and its seasonal
variations. 2) The probability distribution of wind speed and of extreme
wi nds. The mean wind speed must be high enough, and the distribution must
be so that all the data points are very similar. 3) The height variation
of wind speed and wind direction. Wind cannot be too high or too low in
relation to the ground or it is too
difficult to harness. 4) The gustiness of the wind field in both speed
and direction. Gusty winds greatly affect the power output of the
windmills and are usually harmful. 5) The wind direction distribution and
probability of sudden large shifts in di rection. The wind must be
unlikely to suddenly shift direction. It must blow in the same general
direction. 6) the seasonal density of the air, and variations of density
of the air with height. The denser the air, the worse it will be for
windmills. 7) Hazard conditions such as sandstorms, humidity, and
salt-spray, which are bad for windmills. The physics behind these will be
discussed later. 8) Trade winds in the subtropics, and the channeled
wind through mountain passes are especially beneficial to windmills. Once
a suitable location is found, the wind is analyzed extensively, and the
criteria is met, there are still more requisites. 1) The terrain upon
which the windmills are built must be relatively flat. The elevation
difference between the turbine site and the terrain is no larger than 60
meters over a 12-km radius. You may have seen windmills such as those in
California on little hills, but this is because the requirement is met.
The hill may be the only one around for miles. 2) All hills must have
small height to width ratios: h:l must be < 0.016. 3) The elevation
difference between the highest and lowest point must be 1/3 or less of the
height difference between the bottom of the rotor disk and the lowest
point in the terrain strip. The surface roughness of the terrain upon
which the windmill is to be built must be low. If it varies by more than
10%, this is no good. The terrain must be smooth, and consistently so. A
rough surface has more of a negative effect on the wind than a s mooth
surface. There is a value n, called, which is assigned to the terrain in
terms of its roughness. This value is used to calculate the height of the
windmill. For instance, over the sea, the index location, n is 0.14.
Over rough inland country, n is 0.34.
Turbines
Windmills are turbines. The two names can be used synonymously.
Turbines are a means of harnessing the a fluid’s power (the wind) by
converting the kinetic energy of the fluid (the wind) into mechanical
power (the rotating shaft) When the shaft of a w indmill is hooked up to a
generator, electrical energy can be formed. The generator can be used to
produce either DC or AC current. Generators that produce DC can be
connected to batteries, an inverter to produce AC, or to power DC loads.
Some generato rs are connected to heating coils. Generators that produce
AC can be hooked up to AC motors such as water pumps. Windmills are NOT
efficient. At the very most, a windmill can extract only 16/27ths of the
kinetic energy from the wind. This is called the Betz Limit and it can be
mathematically proven through calculus. Most of today’s windmills extract
about 30 perc ent of the wind’s energy. The American farm windmill can
only extract 10%. An important equation used to find the wind power
density, how much power is available per square meter is the equation P =
.5 pu?, where P is the wind power density in W/m2, p is the density of the
air, and u? is the cube of the wind velocity. An equation for the power
available is (kinetic energy flux) = .5 p V3 A, where p is the kinetic
energy density J/m?, V is the velocity of the wind, A is the cross
sectional area of the wind on the turbine.
The equation for determining the power of the shaft, (which is
less than the final power output, since gear trains and generators cause
power to be lost) is as follows: Cp = P((((((((
(0.5 p V ? ( D2)
4
Where Cp is the power coefficient (Power of shaft), p is the air density,
D is the rotor diameter, V is the velocity of the wind and P is the net
power output.
Also Cp = P available
P turbine
The power available is a function of elevation. At ground level, 100% of
the power is available. At 100 feet, 97% is available. At 5000 feet, 86%
is available. Some turbines are shrouded like jet engines. The shroud is
a way to channel the wind. An equation for the power harnessed by a
shrouded wind turbine is: P(Pe) = ( QT ((p + (k) where P is the power,
Pe is the power extracted, ( is the turbine efficiency, QT is the
volumetric flow rate of air on the turbine, (V/A), ((p + (k) is the change
in pressure energy between the inlet and the exit of the wind turbine, and
k is the cane in kinetic energy of a unit volume of air that passes
through the machine.
Shrouds concentrate and diffuse the wind as it passes through a
horizontal access wind-turbine. They reduce the turbulence of the wind
and “direct it”. The advantages of shrouds, as told by Cheremisinoff (pg.
61 of Fundamentals of Wind Energy), are: a ) the axial velocity of the
turbine increases, meaning that smaller rotors can operate at higher
revolutions, b) the shroud can greatly reduce tip-losses, and c) the
aerofoils would not have to be rotated in a direction parallel to the wind
if the wind-di rection changed. The cut in speed is the lowest wind speed
below which no usable power can be produced by a wind turbine. This means
that the wind must be fast enough to move the aerofoils to drive the shaft
to create enough power, after much is lost, so that the end amo unt of
power is greater than zero. Rated power is the maximum power output of a
turbine, which is dependent on a number of factors, especially the
generator. In calculating the height of the windmill, it is important to
keep in mind that the windmill must be high enough to be above
obstructions. The wind velocity decreases as one approaches the surface.
That means that the higher you build, the better chance
there will be that the wind speed is higher, however, you must find the
perfect medium–there are often more variables as you increase in
altitude. In calculating how high a windmill should be the following
equation is used: V1/V2 = (H1/H2)n, Where V1 is the wind speed at the
highest point of the highest blade, V2 is the wind speed at the lowest
point of the lowest blade, H1 is the height of the highest point, and H2
is the height of the lowest point. n is the index location of the site, a
va lue that measures the roughness of the terrain.
The structures, aerofoils (see also vector diagrams, attached)
The support of the windmill is generally made out of steel. The
windshaft is the shaft which carries the windwheel or aerofoils. It is
turned as the aerofoils turn. It is made of steel or wood.
Aerofoils are the blades on a windmill. They can be made out of
any material. They were first made of wood or wood composites. Steel was
used after that. Aluminum is used in the Darrieus windmills because it is
much stronger. Unfortunately, Aluminum fatigues quicker. Some windmills
use fiberglass blades. New materials such as strong alloys are being used
in today’s windmills experimentally. It is important that the blades have
a large lift force and a small drag force. The lift force is the force
needed to bend the flow of the (fluid) air. It is the force perpendicular
to the stream of the air. The drag force is the force parallel to the
stream. The aerofoil must be able to develop a lift force at least 50
times greater than the drag. Torque acts on the aerofoil with a vector
from the center of rotation away. Other forces that act on the blades of
windmills are wind shears, wind gusts, which push on the aerofoils,
gravity, a pull towards the earth, and shifts in the direction of the
wind. Shifts in the direction of the wind are often accounted for by
having a
small blade, called a tailvane, on the backside of a windmill. The wind
blows on a flat side of the tail, which is oriented differently from the
aerofoils. Then, the aerofoils can be rotated to face into the wind. If
the wind is blowing in the directi on of this tail instead of the
direction of the aerofoils, the tail rotates a shaft, which rotates the
whole windmill in the proper direction so as to orient it towards the
wind. As Paul Gipe explained in his book Wind Energy comes of age, (page
27), Wind gusts can greatly affect a windmill. A turbulent gust is a gust
greater than two minutes with a certain mean wind speed. Gusts are
analyzed extensively, with magnitudes, one fo r the lull speed, which is
the wind speed of a negative gust amplitude, and the peak speed, which is
the wind speed for a positive gust amplitude. The gust amplitude is the
difference between the largest speed in the gust and the mean speed. The
gust du ration is the time from the beginning to the end of a gust. The
gust frequency is the number of positive gusts, which occur per unit time.
The gust formation time is the time it takes from the beginning of a gust
to the time it attains the peak gust spe ed. The gust decay time is the
time it takes for the gust the end after it reaches its highest amplitude.
There is quite a bit of terminology with aerofoils. The angle of the
surface to the fluid flow is the angle of attack, alpha. The angle of
attack must be just right. If it is too great, the lift will dramatically
decrease and the drag will increase, st alling the windmill. At rest,
(when the windmill is not in operation), the angle of attack is 85?. When
in motion, the angle of attack is anywhere from 2-10 degrees. Newer and
more advanced windmills have an angle of attack in the upper end of this
ran ge. The pitch angle, ? is the angle between the chord of the aerofoil
and its plane of rotation. The pitch angle can be adjusted. Solidity is
the ratio of the blade width (at widest point) to the distance between the
centers of the blades. A typical “pinwh eel American windmill” might have
a ratio of about 1:1, because the blades are very narrow and very close
together, whereas a new two-bladed aerofoil would have a ratio of about
0.03. There is a transfer of work between the wind stream and the moving
blade. In order for this transfer to be efficient, a typical blade is
usually 1/4 the width of its length. (If the blade is 10 feet long, it
will be 2.5 feet wide at its widest point). A erofoils come in many
shapes. Some blades are made a little wider than this ratio, because it
is easier to start such a windmill. However, blades like this aren’t as
efficient. No matter what the shape, “most have a blunt nose and a finely
tapering tab le” (Calvert). A flow must be able to follow the curved
surfaces of the aerofoil without being separated. The mass flow rate is
given by the equation: m = p Vb A, where p is the air density, Vb is the
air speed at the blades and A is the area. The number of blades on a
windmill varies. There are many different types of windmills. The
following equation helps figure out how fast the a certain-bladed windmill
will rotate in relation to windmills with different numbers of blades:
Speed of windmill = 1 / sq. root of number of blades The aerofoils of a
four bladed machine rotate 71% as fast as that of a 2 bladed machine. A
six bladed machine rotates at 58% and an 8 bladed machine rotates at 56%
as fast as a 2 bladed machine.
Electricity and Storage of Energy
As mentioned previously, the generators in a wind turbine can
convert the mechanical energy produced by the rotation of the shaft into
electrical energy, DC. From there, some windmills have synchronous
inverters, complex electronic devices which convert
the DC generated by the turbines into AC. This is an expensive option.
There is a loss of power as well through its processes. Others have
induction generators, which produce AC current without a synchronous
inverter and less power loss. The energy extracted from the wind and
converted into mechanical energy then electrical energy by the generator
must be stored, since it is not used generally used all at once. It is
important to keep a surplus of energy for usage when the wind is not bl
owing fast enough, despite the corrections that can be made in the pitch
of the aerofoil blades and when the windmill is out of service or the
demand is especially high.
Storing the wind’s energy effectively is the key to its long-term
use. Windmills used as water pumpers or air-compressors can pump excess
water, hydrogen or air into reserve tanks. Today, there are a number of
ways to store the wind’s energy. Windmills
are used to charge Electrolyte batteries. Lead-acid or Lead-cobalt car
batteries are commonly used as well. However, batteries may be expensive
and inefficient–they may lose 10-25% of the energy stored in them.
Nickel-Iron, Nickel-cadmium, and zinc-a ir cells are often used as well.
These tend to be more efficient. Some windmills are now using organic
electrolyte batteries such as CuCl2, Ni Cl2, and NiF2 batteries as well as
sodium-sulfur batteries, which operate at high temperature, are used.
Although uncommon and still in experimental phases, some energy is
stored not by being converted directly into electrical energy, but rather
by being stored as thermal or electromagnetic energy,
Sound Fluids are elastic. Pressure waves are constantly being created and
propagated by the aerofoils and the turbine as a whole (entire components
excepting the support). We can hear them in the sound given off. The
sound intensity is directly proportional with the speed of the windmill.
The frequency of the waves is directly proportional to the angular speed
of the blades on the rotor. The flutter you hear has aerodynamic and
elastic properties. The higher speed the aerofoils are, the louder the
sound a nd the louder the flutter they will make, as more pressure waves
are being created and propagated. The generators are noisy. They often
confuse birds and cause them to fly towards the turbine. Windmills can be
very noisy. A 300 kW turbine at 1 mile away has a dB level equal to a
traffic light 100 feet away (Gipe). Windmill sound levels are regulate d.
The sound level must be kept under 46 dB in a residential area. Wind
turbines can cause interference, disturbances with TV and radio reception
(ghost images on TVs), affect microwaves and disrupt satellite
communication. These problems are currently being resolved. Many have
already been fixed. There is also a .009
probability of a bird or insect being struck by the blades. Windmill